Methylation of bases in DNA serves a number of cellular functions. In bacteria, methylation of cytosine and adenine residues plays a role in the regulation of DNA replication and DNA repair. DNA methylation also constitutes part of a immune mechanism that allows these organisms to distinguish between self and non-self DNA. In mammalian species, DNA methylation occurs only at cytosine residues, and specifically at cytosine residues that lie next to a guanosine residue, i.e., within the sequence CG. Methylation of DNA is carried out by methyltransferases (sometimes called methylases). Generally both DNA strands can accept methyl groups at opposing CG sites. Replication of these strands yields a hemi-methylated state which is recognized by a class of maintenance methyltransferases capable of restoring full methylation to both strands. Most CG sites in the genome are methylated except for those in CpG ‘islands’ which remain methylation-free. CpG ‘islands’ are rich in CG sites and are often found near coding regions within the genome (i.e., genes). About half of the genes in the human genome are associated with CpG islands. Importantly, the vast majority of CpG islands in the genome remain unmethylated in normal adult cells and tissues. Methylation of CpG islands is normally seen only on the inactive X-chromosome in females and at imprinted genes where it functions in the stable silencing of such genes. Strict control over the levels and distribution of DNA methylation are essential to normal animal development.
Alterations in DNA methylation are one manifestation of the genome instability characteristic of human tumors. A hallmark of human carcinogenesis is the loss of normal constraints on cell growth resulting from genetic alterations in the genes that control cell growth. The consequences of such mutations include the activation of positive growth signals and the inactivation of growth inhibitory signals. Gene function can be lost through mutation or deletion. An alternative mechanism by which gene function can be lost is aberrant DNA methylation. Accordingly, such methylation events can be viewed as key steps in both the initiation and progression of neoplastic disease.
Identification of gene targets which when methylated lead to the loss of normal cell responses, as well as identification of agents which desirably prevent or at least control such methylation events would be valuable. Such gene targets and agents will facilitate the diagnosis and treatment of disorders associated with abnormal methylation and any downstream events resulting therefrom.